1. Field of the Invention
The present invention relates to multiplex transmission methods and systems, and to audio jitter absorbing methods used therein, and more particularly to a method of multiplexing and transmitting low-bitrate-compressed audio data and video data and/or computer data and a method of absorbing audio jitter caused in the transmission.
2. Description of the Background Art
In visual telephones and video conference systems, video signals having enormous amounts of information are generally low-bitrate-coded and then transmitted to the destination. H.263 in the draft stage of the international standardization by ITU-T (International Telecommunication Union-Telecommunication Standardization Sector) is known as an example of such low bit rate coding. These standardizations adopt the motion compensative interframe predictive coding and the variable-length coding as a compression coding method, which are very sensitive to transmission errors. That is to say, when an error, even of one bit, occurs, the influence of the error transfers in a large area of a picture and also transfers in the time axis direction, providing serious deterioration of the image quality. Hence, in the analogue telephone line which is prone to transmission errors, error correction by retransmission control is generally performed before image decoding.
On the other hand, as shown in the draft G.723 issued by ITU-T, audio data are generally compressed to a very low bit rate by low bitrate coding when transmitted. In the audio data, transmission errors are not so noticeable as in the video data. However, it is sensitive to the transmission delay. Hence, according to G.723, when an error is detected, muting is performed without error correction by retransmission, so as to prevent the transmission error from causing noise.
As a method for multiplexing and transmitting video data, or variable-length data employing retransmission, and audio data, or fixed-length data not employing retransmission, ITU-T has also issued draft H.223.
H.223 will now be explained referring to the drawings as an example of a conventional multiplex transmission method.
FIG. 44 shows a general layer structure of a multiplex transmission device adopting the multiplex transmission method determined by H.223. In FIG. 44, this multiplex transmission device includes a physical layer 801, a multiplex layer 802, an adaptation layer 803, a video coder 804, an audio coder 805, a data protocol 806, an LAPM 807, an H.245 control 808, a video I/O 809, an audio I/O 810, and an application layer 811.
H.223 supposes a modem for analogue telephone line at 28.8 Kbps as the physical layer 801. It defines, as the multiplex layer 802 and the adaptation layer 803, a frame structure and a procedure for video error correction by retransmission control and a frame structure for audio error detection. Supposed as the video coder 804 is H.263. As the audio coder 805, G.723 is supposed. ITU-T issues H.324 as a specification recommendation for the entire system. H.223 does not specifically define the data protocol 806. A communication procedure for control data is defined as the LAPM 807. As the H.245 control 808, commands and procedure for system control are specified.
Next, FIG. 45 shows the multiplex frame format in the multiplex layer 802 of H.223. In FIG. 45, for the opening flag and the closing flag, the same bit pattern "01111110" as that used in HDLC is used to establish flag synchronization. In order to keep the transparency of the opening flag and the closing flag, H.223 defines to insert 0 after five contiguous 1's in a part other than a flag and to remove it on the receiving end. The header shows, into which types of slots the following information field is divided and which types of data are multiplexed. FIG. 46 shows an example of a multiplex frame format in the multiplex layer 802 with audio data, computer data and video data multiplexed.
Defined in the adaptation layer 803 are a frame format and a procedure for error correction of video by retransmission control. FIG. 47 shows an ARQ (Automatic Repeat reQuest) frame format for error correction of video by retransmission control. In FIG. 47, the control field contains a transmission number for retransmission control and a flag indicating whether the following payload field is an information frame or a supervisory frame. The payload field contains video data in the case of an information frame and contains a retransmission request number and a retransmission request command in the case of a supervisory frame. The CRC field contains a check code for error detection. The procedure of retransmission control determined in the adaptation layer 803 is the selective repeat ARQ method of a window size of 128 frames, in which a retransmission is requested only once in a supervisory frame by using the transmission number of a frame with an error detected by the CRC check as the retransmission request number, together with the retransmission request command.
The H.223 gives variable lengths both to multiplex frames and video frames and uses flag synchronization. Accordingly, frame synchronization is lost if a flag has an error. When an error occurs in other data, that data may simulate itself as a flag to break the synchronization. The H.223 thus has the problem of being easily affected by error.
The radio channel of the PHS (Personal Handy-phone System) which is a simplified portable telephone (which has a bit rate of 32 Kbps) is still more prone to transmission errors than the analogue telephone line. Conventionally, as a, transmission control method used when transmitting computer data through the radio channel of PHS, PIAFS (PHS Internet Access Forum Standard) is suggested within the country of Japan, for example.
An ARQ frame in PIAFS has a fixed length (80 bytes). FIG. 48 shows a frame structure of PIAFS. In FIG. 48, the frame type identification field contains a frame type, the error control field contains a transmission frame number and an oldest unreceived frame number, the user data length indication field contains the data length of significant data in the user data field, the user data field contains user data, and the error detection code field contains an error detection code for detecting error of the entire ARQ frame.
In the PIAFS, frame synchronization is established prior to a communication by using a synchronizing frame including a synchronization flag and the frame synchronization is kept by using data frames having the same length as the synchronizing frame, which requires no synchronization flag in the data frames. The PIAFS, with fixed-length ARQ frames, is not susceptible to error because the frame synchronization is not lost even if an error occurs.
Hence, if fixed-length multiplex frames are used in H.223, the multiplex frames can be synchronized even if an error occurs, which will provide the multiplex frames with a high tolerance for errors. Japanese Patent Publication No.8-13057 discloses, an example thereof, as "Mixing Transfer System for HDLC Variable Length Packet and Non-HDLC Fixed Length Packet." The transfer method disclosed in the reference will now be explained referring to FIG. 49.
In FIG. 49, each fixed-length frame is provided with a header showing whether the contents are HDLC data or not. When the header indicates non-HDLC data, the fixed-length frame contains a non-HDLC fixed-length packet (e.g., an audio data packet). When the header indicates HDLC data, the fixed-length frame contains part of a divided variable-length HDLC frame (e.g., a video frame). According to this method, the multiplex frames, as having a fixed length provide error robustness. However, the video data still adopt the HDLC flag synchronization and are stored over a plurality of multiplex frames. Accordingly, the video frames are still easily affected by error.
For the purpose of taking advantage of the function of increasing transmission band width for video data or the like by performing silence detection to produce no audio frame when silent, as in G.723, it is necessary to suppress jitter under several mili-seconds. This is due to the fact that if jitter is large, the receiving side can not tell whether audio data is not received due to jitter or audio data is not received because it is in a silent period. Then suitable processing can not be performed.
Conventionally, as a method for minimizing jitter of fixed-length cell packets, a method of comparing a predetermined threshold and the number of cells in a buffer having a capacity twice the maximum fluctuation time to control reading of the buffer is suggested. (Refer to Japanese Patent Laying-Open No.5-244186 "ATM/STM Converting Circuit.") FIG. 50 shows the structure thereof. In FIG. 49, the buffer 50 has a capacity of 2.DELTA.t, twice the jitter .DELTA.t. The reading control portion 60 compares the number of cells accumulated in the buffer 50 and a predetermined threshold. When the number of accumulated cells exceeds the threshold, it outputs a reading address to start reading the buffer 50. Thus, the jitter correcting circuit of FIG. 50 is so structured as to read contents of the buffer 50 after a maximum delay time in the ATM network has passed after arrival of the first cell. However, when the jitter correcting circuit of FIG. 50 is used, the average delay time increases as the cells are accumulated to the threshold, increasing delay of audio.
When the multiple header has an error, the frame number of the video frame multiplexed in the discarded information field can not be known and then a retransmission request can not be immediately transmitted to the transmitting side. Generally, the multiplex layer discards data in the information field when an error is found in the multiplex header. Accordingly, it is when the sequence error is detected from the frame number of the ARQ header of the video frame received next without error that a retransmission request for the video frame in the discarded information field is sent out from the error control processing layer. For example, if a transmission error occurs in the multiplex header of the multiplex frame of the frame number (1) transmitted from the transmitting terminal, the receiving terminal discards video data in the multiplex frame after receiving the multiplex header. Accordingly, the receiving terminal can not immediately send a reject for the video data of the frame number (1). That is to say, the receiving terminal finds the absence of the multiplex frame of the frame number (1) only after it receives the multiplex frame of the frame number (2), and then it transmits a reject for the frame number (1) to the transmitting side.
When the error detection for the frame numbers is performed independently of the error detection for the information fields as shown in the partial repeat ARQ for mobile packet communications presented in Electronic Information Communication Society, National Meeting in Spring, B-323, 1990, the receiving side can, even if the information part is discarded, immediately transmit a retransmission request for the frame as long as the frame number of that frame correctly arrives at the receiving side. This reduces the retransmission waiting time.